Electrochemical cells comprising polymer electrolyte membranes (PEMs) may be operated as fuel cells wherein a fuel and an oxidant are electrochemically converted at the cell electrodes to produce electrical power, or as electrolyzers wherein an external electrical current is passed between the cell electrodes, typically through water, resulting in generation of hydrogen and oxygen at the respective electrodes of the cell. FIG. 1 illustrates a typical design of a conventional electrochemical cell comprising a PEM, and a stack of such cells. Each cell comprises a membrane electrode assembly (MEA) 5 such as that illustrated in an exploded view in FIG. 1a. MEA 5 comprises an ion-conducting PEM layer 2 interposed between two electrode layers 1,3 which are typically porous and electrically conductive, and comprise an electrocatalyst at the interface with the adjacent PEM layer 2 for promoting the desired electrochemical reaction. The electrocatalyst generally defines the electrochemically active area of the cell. The MEA is typically consolidated as a bonded laminated assembly. In an individual cell 10, illustrated in an exploded view in FIG. 1b, an MEA is interposed between a pair of separator plates 11, 12, which are typically fluid impermeable and electrically conductive. The cell separator plates are typically manufactured from non-metals such as graphite or from metals, such as certain grades of steel or surface treated metals, or from electrically conductive plastic composite materials. Fluid flow spaces, such as passages or chambers, are provided between the plate and the adjacent electrode to facilitate access of reactants to the electrodes and removal of products. Such spaces may, for example, be provided by means of spacers between separator plates 11, 12 and corresponding electrodes 1, 3, or by provision of a mesh or porous fluid flow layer between separator plates 11, 12 and corresponding electrodes 1, 3. More commonly channels (not shown) are formed in the face of the separator plate facing the electrode. Separator plates comprising such channels are commonly referred to as fluid flow field plates. In conventional PEM cells, resilient gaskets or seals are typically provided between the faces of the MEA 5 and each separator plate 11, 12 around the perimeter to prevent leakage of fluid reactant and product streams.
Electrochemical cells with a ion-conductive PEM layer, hereinafter called PEM cells, are advantageously stacked to form a stack 100 (see FIG. 1d) comprising a plurality of cells disposed between a pair of end plates 17, 18. A compression mechanism (not shown) is typically employed to hold the cells tightly together, maintain good electrical contact between components and to compress the seals. In the embodiment illustrated in FIG. 1c, each cell 10 comprises a pair of separator plates 11, 12 in a configuration with two separator plates per MEA. Cooling spaces or layers may be provided between some or all of the adjacent pairs of separator plates in the stack assembly. An alternative configuration has a single separator plate or "bipolar plate" interposed between pairs of MEAs, contacting the cathode of one cell and the anode of the adjacent cell, thus resulting in only one separator plate per MEA in the stack (except for the end cell). The stack may comprises a cooling layer interposed between every few cells of the stack, rather than between each adjacent pair of cells.
The cell elements described have openings 30 formed therein which, in the stacked assembly, align to form fluid manifolds for supply and exhaust of reactants and products and, if cooling spaces are provided, for a cooling medium. Again, resilient gaskets or seals are typically provided between the faces of the MEA 5 and each separator plate 11, 12 around the perimeter of these fluid manifold openings to prevent leakage and intermixing of fluid streams in the operating stack.
In the future it is anticipated that a major area of application for PEM fuel cells, will be for electrical power generation in stationary power plants and portable power generation systems, and for propulsion in motor vehicles. For these applications, a PEM fuel cell service life of at least 10 years is desirable. Production costs are important and will play a central role in the successful commercialization of PEM fuel cells for these applications. Other important considerations when designing a PEM cell are simplicity and cost-effectiveness of maintenance and repair.
The present invention relates to improved sealing and construction of individual PEM cells and stacks of such cells. Conventional PEM cell sealing mechanisms generally employ resilient gaskets made of elastomeric materials, which are typically disposed in grooves in the separator plates or MEAs, for example, as described in U.S. Pat. Nos. 5,176,966 and 5,284,718. Over the course of an electrochemical cell's service life the elastomeric gaskets are subjected to prolonged deformation and sometimes a harsh operating environment. Over time such gaskets tend to decrease in resilience, for example due to compression set and chemical degradation, and may become permanently deformed. This impacts negatively on the sealing function and can ultimately lead to an increased incidence of leaks.
With such gasketed plates, the plastic deformation of the plates increases as the full force of pressure on the sealing area of the plate is continuously applied. Moreover, an uneven gasket pressure force distribution along the length of the stack, with a minimum in the center, can be observed in stacks using such a sealing mechanism. Thus, the sealing elements of the cells are typically exposed to higher pressure in the end plate areas in order to guarantee adequate sealing performance in the center cells of the stack. Increased sealing pressure applied to the cells in the end plate areas may then lead to increased plastic deformations and a shorter time to gasket failure.
The assembly of a PEM cell stack which comprises a plurality of PEM cells each having many separate gaskets which must be fitted to or formed on the various components is labor-intensive, costly and generally unsuited to high-volume manufacture due to the multitude of parts and assembly steps required. Further, in the design and manufacture of PEM cells, in order to achieve the desired specifications, such as increased power density, there is a desire to make the individual cell elements thinner. Accordingly, there will be finer dimensional tolerances required for such thin cell elements and it will become more difficult to design gaskets which will maintain high dimensional tolerances, despite the use of highly elastic materials, as even highly elastic materials have a limited elastic deformation range.
With conventional PEM cell designs, it is sometimes difficult to remove and repair an individual cell or to identify or test which cells in a stack may require repair. Furthermore, disassembly of a stack consisting of multiple cells each comprising separate cell components can be very costly as in many instances, after the removal of one cell, the gaskets of all the remaining cells may need to be replaced before the stack can be reassembled and reused.
Another disadvantage of conventional PEM cells arises because the PEM typically projects beyond the edges of the electrodes and cell separator plates around the perimeter and around manifold openings. The projecting portion of the PEM may serve to avoid short circuits between plates, and it typically contacts and cooperates with the gaskets to form the fluid seal between the MEA and separator plates. However, such designs tend to leave the PEM edge exposed to air and/or reactant or coolant streams. Exposure to air or other dry gas streams can cause drying of the PEM beginning from the edge and moving towards the center. Drying of the membrane can lead to permanent damage to the membrane, reduced cell performance and ultimately malfunction of the PEM cells. Exposure of the PEM edge to some coolants and other streams can result in physical and/or chemical damage to the membrane or electrodes.
German printed patent application number DE 44 42 285 describes a PEM cell stack where individual components of the stack are mechanically pressed together with a frame element or clamp of U-shaped cross-section. Specifically, the edge portions of two separator plates are pressed against the membrane, which is disposed between them, to form a gas-tight seal.
The European published application EP 0 331 128 and related U.S. Pat. No. 4,786,568 disclose a liquid electrolyte fuel cell with a porous electrode substrate which has a sealant material injected into a marginal edge portion thereof.
EP 0 122 150 describes a fuel cell stack with liquid, fixed electrolytes. In this stack array, two adjacent, porous and gas-bearing plates are bonded or glued together across their entire surface, in a face-to-face bond. The purpose of this arrangement is to keep the gases in the two gas-bearing plates separated from one another.
In the EP 0 083 937 and related U.S. Pat. No. 4,397,917, individual components of a fuel cell stack are glued together with an adhesive material. The purpose of this arrangement is to solidly join the components to form a stack, and not to reliably seal the gas spaces.
DE 19 64 811 discloses a fuel cell stack where the electrodes are attached inside a sectional plastic frame, respectively, and where the sectional frames are glued together in a gas and liquid-tight manner.
An improved electrochemical PEM cell uses an adhesive bonding agent between individual PEM cell components and/or between adjacent PEM cells in a stack. The present approach provides a simplified PEM cell and stack design with a reduced part count, and associated manufacturing and cost benefits. Sealing is generally more reliable with this approach, and embodiments of the present construction may permit easier stack disassembly, testing, repair and maintenance.